A study33 fed one group of rats a diet that included yogurt sweetened with sugar, and another group of rats a diet that included yogurt sweetened with a zero-calorie artificial sweetener commonly found in diet cola. The rats that were fed a zero-calorie sweetener gained more weight and more body fat compared to the rats that were fed sugar. After the study was published, many news articles discussed the implication that people who drink diet soda gain more weight. Explain why we cannot conclude that this is necessarily true.

this effect has only been demonstrated in model animals (in this case rats) and therefore the outcome of this research cannot be generally extended to humans

Explanation:

In the last centuries, experimental animals have been extensively used as research models in order to assess the possible outcomes in humans. In this regard, it is important to highlight that almost all medical development in use today (e.g., vaccines, medical drugs, therapies, etc) were first developed and tested in animal models before being used in humans. However, animal models are not humans. In this case, rats obviously don't have the same metabolic pathways (and food habits) as humans, thereby results cannot be translated directly to humans. In consequence, this study might be considered as a proof of concept, but further research is needed to determine the effects that drinking diet soda may have on human weight.

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between the two muscle layers is the myenteric or auerbach's plexus, which controls peristalsis. in the colon, the muscularis externa is much thicker because the feces are large and heavy, requiring more force to push along. the stomach has a third layer of muscularis externa: the inner oblique layer.